1. Field of the Invention
The present invention generally relates to dielectric measurements on thin materials and, more particularly, to a probe assembly and an apparatus and method for making measurements of the dielectric constant of microwave circuit board or substrate materials over very small areas.
2. Description of the Prior Art
Using the simplest definition, the relative permittivity, .epsilon..sub.r, of a material can be derived from the alignment of dipoles between charges in a dielectric medium. More specifically, if an electric field is applied to a dielectric material, an atomically small displacement of positive and negative charges, or polarization, occurs within each dipole. Each dipole has, in effect, produced a field that reduces the total electric field strength in the system. The effective magnitude of the electric field contributed by each charge to the total electric field is reduced by a factor, .epsilon..sub.r.sup.-1. The relative permittivity (or dielectric constant), .epsilon..sub.r, is a resulting characteristic of the physical structure of the medium in which the electric field exists.
Transmission line theory is directly affected by the dielectric properties of a medium. The techniques and technology used in making microwave circuits have presented the need to verify the dielectric constant in a material so that predictable and expected results occur from the fabricated circuit. Experimentally determining a material's dielectric constant has taken several different measurement approaches. Some examples are measurements using a resonant cavity, resonant transmission line sections etched on the substrate, resonant half-height waveguide sections, and free space transmission phase delay.
These methods give measurements of the dielectric constant which are averaged over areas of approximately one-half wavelength squared at the frequency of measurement. The main limitation of these methods is that they either require destructive testing, or the results are an average over a relatively large area. These methods have proved unusable for performing measurements on small areas of typical microstrip fabricating materials (the thickness ranging from 10 to 35 mil) at the frequencies of 2 to 18 GHz.
Consequently, there is a need for a different approach to measurement of the dielectric constant of thin flat sheets of microwave substrate materials.